In CMOS integrated circuits, the design of functional blocks are often designed in a differential form due to the imperfect ground reference lacking a thru via from the top layer to the backside of CMOS substrate. Therefore, a single-ended block, for example, an RF power amplifier, requires conversions from a single-ended port to differential ports or differential ports to a single-ended port, often represented balanced-to-unbalanced (balun). Since the conversion mainly comes from the magnetic coupling from the balanced ports to the unbalanced port, the conversion does not share the same reference. In an output matching of a power amplifier, a vaguely defined reference of the unbalanced output port often causes the loss of the balun conversion. Therefore, a reference enhancement at the unbalanced output port is desirable to minimize the loss of conversion, in other words, to enhance the efficiency in the power amplifier.
FIG. 1 illustrates a block diagram of a conventional power amplifier system 100 with a transformer output matching. In FIG. 1, a transformer 107 is oftentimes used at the output of a CMOS power amplifier (PA) 101 for impedance matching and balun function. While the function of impedance matching helps provide high output generation at the output port, the additional balanced-to-unbalanced conversion often generates more loss. The loss does not come only from the quality factor of the transformer 107, but also from the lack of an exact common reference for the unbalanced port.
FIG. 2 illustrates an impact of noise in a conventional power amplifier system 200. As shown in FIG. 2, the convention transformer 220 has a primary winding 204 and a secondary winding 205. The conventional transformer 220 has a first reference point 206 in the primary winding 204 and a second reference point 207 in the secondary winding 205. Parasitic elements 208, 209, 210 can form an imperfect and inconsistent parasitic network that allows some limited communication between the first reference point 206 and the second reference point 207. However, the references points 206, 207 are not guaranteed that to be equal in electrical reference levels due to the imperfect and inconsistent parasitic network provided parasitic elements 208, 209, 210. Indeed, if any noise 212, 214 is present on both reference points 206, 207, the coupling between the primary winding 204 and the secondary winding 205 fluctuates, as shown by a comparison of PA output signal 206 and transformer output signal 213. The fluctuation in coupling is more severe in large signal operation like a power amplifier with bonding wires for imperfect reference.